The present invention relates to fabrication and repair of gas turbine engine components and the like, and more particularly, to a novel method for opening partially or substantially completely blocked film cooling holes in a gas turbine engine component or the like, after application of a thermal barrier coating (TBC).
Static and dynamic components of a gas turbine engine which operate in a hot section of the engine are typically coated with a thick coating of the thermal barrier type, such as Zirconia oxide or the like. The thickness of the thermal barrier coating is currently limited or restricted because thicker application of the coating can result in full or partial closure of the cooling holes formed in some gas turbine engine components, particularly those located in the hot section of the engine, to extend the operating life of the component and to prevent heat damage to the component during engine operation.
Referring to FIG. 1, as an example of a gas turbine engine component having film cooling holes formed therein and also being coated with a TBC, a gas turbine engine blade or airfoil 10 is shown having a multiplicity of cooling channels or holes 12 formed therein to permit cooling of the blade during engine operation. The cooling holes 12 extend from an exterior surface 14 of blade 10 into a hollow interior 16 or plenum of the blade 10. The blade hollow interior includes a plurality of interior walls 18 or baffles to direct cooling air indicated by arrows 20, through the interior of blade 10 and out cooling holes 12 to create cooling air streams, indicated by arrows 22. Cooling air 20 absorbs heat within the interior 16 of blade 10 and also from the walls surrounding cooling holes 12 and the cooling air streams 22 exiting cooling holes 12 flow over the exterior surface 14 to further cool the blade 10.
In order to function properly, the cooling holes 12 must be constructed to a specified configuration and dimensions because the distribution of air flow must be controlled to achieve proper cooling of the blade 10 during engine operation. Thus, the cooling holes 12 must not be blocked or even partially blocked to provide sufficient and uniform cooling air distribution through the blade interior 16 and across the exterior 14 of blade 10. Application of a TBC with a thickness of about 5 mils or more can result in a significant reduction in air flow through cooling holes 12 and a TBC thickness of about 15 mils or more can result in complete closure of the cooling holes 12. Therefore, to apply a TBC coating of more than about 5 mils, at least some of the cooling holes 12 may have to be opened to their original specified dimensions to provide proper cooling of the component 10 and the holes 12 must be opened without causing damage to the parent material of the component 10 or to the TBC in areas other than where it is desired to remove the TBC.
The present invention accomplishes the removal of the TBC in desired areas by use of an Excimer laser (ultraviolet laser).
The use of Excimer lasers for material processing, such as micro-machining and deposition of metallic and insulator films is described in marketing and technical literature published by Lambda Physik GmbH, a leading manufacturer of Excimer lasers, whose address is Hans Bockler-Str. 12, D-3400 Gottingen, Federal Republic of Germany, and in U.S. Pat. Nos. 4,617,085; 4,756,785; 4,720,621; 4,368,080; 4,328,410; 4,219,721; and 4,128,752. None of these documents recognize the specific problems, as mentioned hereinabove, associated with the removal of TBC material from the cooling holes of a gas turbine engine component or suggest the solution as provided by the present invention.
Other presently known material processing methods, such as abrasive grit drilling, Nd:YAG laser drilling, CO.sub.2 laser drilling, water jet drilling and the like, have disadvantages which make them inappropriate for use in opening cooling holes. Abrasive grit drilling is slow and inefficient in a high capacity production environment and may present control problems in drilling a precise opening with minimal damage to surrounding areas. YAG and CO.sub.2 laser drilling involves thermal removal of the material which can result in damage to the parent material of the component and to the TBC material surrounding the cooling hole being opened. Water jet drilling is also difficult to precisely control to open the cooling hole and avoid damage to the component.